Sparring is a form of training like any other form of training. Even the most brutal MMA matches are not fights. They are sparring.

As with any form of training, the questions are: What does it get you? What does it cost you? Every kind of sparring, from light to no contact “tag” to MMA matches serves a purpose.

Tag type sparring (light to no contact)
What you get: You learn control. You learn to put your fist or foot or elbow where you want it to go.
What it costs: Very little.
The non-physical dangers: But the student has to be certain they understand this is far, far from any kind of actual fight. So don’t get cocky because you’re the school “tag” champion.

Kickboxing
What you get: You improve your stamina and strength. You learn range, speed, combinations, positioning and…you learn to take a heavy hit (or several) without stopping.
What it costs: You can expect to always get bruised up, and occasionally there will be more serious injuries.
The non-physical dangers: For safety reasons there are lots of rules with kickboxing. These limitations on what you (and your opponent) can do make this EXERCISE a very unrealistic imitation of a fight. As before, don’t get cocky because you can do this well.

Grappling & ground fighting
What you get: Practical experience and a “feel” for grappling, joint locks, throws and so on, for people of different weights and sizes.
What it costs: Like with kickboxing, expect bruises and abrasions. And, unfortunately, the occasional injury.
The non-physical dangers: Some Jujitsu consider themselves to be the toughest guys around. That doesn’t count for much if you’re ground fighting your assailant, but his friend is kicking you. As will all sparring, it’s an exercise.

With any of these exercises (and many other variations), always remember what it gets you, what it costs you and remember the non-physical danger that you might start to think that’s how fighting works.

Whether any of these is worth the risk, that depends on each practitioner, and how prepared they want to be if they’re attacked, and what risks they’re willing to take for that level of preparation.

In many cases the more dangerous kinds of sparring can be done for a while, until those particular lessons are learned, and then one can move on.

Early on, one of the most valuable things that a beginner can learn in sparring is that you CAN continue even if you have the wind knocked out of you or a charlie-horse. That is a critical lesson to learn for self defense. If you don’t learn it, should someone unexpectedly hit you hard in real life, you’ll crumple. You won’t know if you are hurt badly or not. All you’ll know is that you’re hurt more than you’ve ever had to deal with before. And when your life depends on it is NOT when you want to learn to handle that.

Once you HAVE learned that (painful) lesson, there’s no need to continue getting the stuffing beaten out of yourself.

As for sparring competition, that’s another matter. It’s not a bad thing to spar for sport. No worse than playing football or rugby. But it’s not about preparing to defend yourself. It’s just a different animal.